A varactor diode is a diode having a variable capacitance, which may be modulated by an external control signal, which is typically in the form of a voltage signal applied to a control gate. The capacitance of the varactor diode may be varied by an external voltage bias that expands or shrinks a depletion region at a p-n junction.
The variability of the capacitance in a varactor may be advantageously employed in various electronic circuits to provide useful functions in amplifiers, oscillators, and frequency synthesizers. For example, varactors may be used to construct voltage-controlled oscillators (VCO), which generates a tunable stable frequency without employing a circuit with multiple oscillators. A VCO is a versatile basic building block for constructing transceiver circuitry, phase locked loop (PLL) circuitry, and other wireless communication circuitry.
The p-n junction of a varactor diode is reverse-biased to suppress current flow though the p-n junction. The capacitance of the varactor diode is varied with the control voltage as the volume of the depletion zone varies with the applied control voltage. Typically, the capacitance is inversely proportional to the thickness of the depletion region.
Metal-semiconductor-oxide (MOS) varactors employing a planar configuration are known in the art. One such example is a metal-oxide-semiconductor field effect transistor (MOSFET) varactor formed in a semiconductor substrate. While such a MOS varactor provides adequate performance in a bulk substrate, a MOSFET varactor formed in a top semiconductor layer in a semiconductor-on-insulator (SOI) substrate suffers from a higher leakage current than a bulk counterpart. As the thickness of the gate dielectric scales down, the increase in the leakage current compromises the effectiveness of a MOS varactor.
Further, formation of a varactor diode including a hyperabrupt junction in an SOI substrate is very difficult due to the limited thickness of the top semiconductor layer since formation of a hyperabrupt junction requires multiple ion implantation steps with different implantation depths. Since advanced semiconductor devices employ an SOI substrate having a thin top semiconductor layer, of which the thickness is typically less than 100 nm, and frequently less than 50 nm, the thickness of the top semiconductor layer is insufficient to form a hyperabrupt junction, or a sharp clearly-defined p-n junction including an abrupt transition in the doping profile with the distance from the junction surface. Thus, a high quality varactor diode is difficult to form on an SOI substrate including a thin top semiconductor layer employing conventional semiconductor structures.
In view of the above, there exists a need for a semiconductor structure providing a hyperabrupt junction within a thin top semiconductor layer of a semiconductor-on-insulator (SOI) substrate, and methods of manufacturing the same.
Particularly, there exists a need for a varactor diode including a hyperabrupt junction formed on an SOI substrate, and methods of manufacturing the same.